Investigation of an Axisymmetric Spiked Body in a Hypersonic Shock Tunnel
The forefront research in the regime of hypersonic flow is intended to focus on improving the overall performance of the existing aerospace vehicles. A high-speed vehicle experiences a high level of aerodynamic heating and drag. These vehicles are also characterised by shock impingement on the body and shock-shock interaction closer to the body surface, which cause pressure and thermal loading. With the addition of a spike on the main body, the flow field over high-speed vehicles can be changed drastically, which would create an impact on the heat flux and aerodynamic drag at hypersonic speed. The aerospike with a hemispherical blunt body and the sharp tip spike over a flat-faced cylinder are two types of spike configurations with different flow phenomenon that are studied in this investigation. The first part of the study focuses on exploring the possibility of addressing aerodynamic drag and heat transfer on a body by placing an aerospike at the nose (i.e., stagnation region) of the high-speed vehicle and as a result, wave drag of a vehicle is considerably reduced. However, this can lead to an increase in heat transfer at localized spots of the main body. An attempt is made in the current study to reduce these localized high heat transfer spots and evaluate the variation in both heat transfer and drag on the body, by modifying the spike configuration through the addition of a smaller hemisphere midway along length of the aerospike. In this regard, shock tunnel experiments and computational studies are carried out on this modified spike configuration, termed a double-disk spike or double spike. The experimental results show that the heat transfer rate near the localized spot of the blunt body with spike decreases for a double spike in comparison with a single disk spike. The decrease in heat transfer varies from 5% to 30% depending on the double spike configuration (i.e. varying cap radius and length of the spike). To supplement these results, the 3D Finite Volume solver, HiFUN (High-Resolution Flow Solver on Unstructured mesh) is used and it showed a good trend in heat transfer and drag compared with experiment. In the second part, an investigation is carried out to understand the unsteady flow phenomenon exhibited by certain types of spiked bodies (Pulsation and Oscillation) which are characteristics by a shock-shock interaction, which in turn depends on spike length and blunt body shape. This unsteady flow separation leads to pressure and heat transfer fluctuations of high amplitude and frequency. Pulsation is characterised by the formation of a separation zone at the spike with a conical shock engulfing it, which grows in size and collapses, and this cycle continues. Pulsating flow is observed for shorter spike length (0.2< L/D < 1.25). Oscillations for the longer spike (1.25 < L/D<2.5), are identified by a small change in the size of the conical separated flow region and an oscillating conical shock engulfing the body. An attempt is made to measure heat flux for these types of unsteady flows, which would provide additional insight and lead to a better understanding of the flow phenomena. Experiments are carried out on a sharp-tipped spike attached to a flat-faced cylinder in a shock tunnel at Mach 5.7. Provision is made to vary the spike length, with L/D = 0.75, 1, 1.25, 1.5 and 2 and results are discussed in detail. The measured heat transfer using a platinum thin-film sensor varied around 100 W/cm2 to 10 W/cm2 for a pulsation case (L/D = 0.75). For the oscillation case all except the corner gauge, indicated the heat transfer varied between 50 W/cm2 and 5 W/cm2. Last gauge shows higher heat transfer values of around 120 W/cm2 to 10 W/cm2 , indicating the shock interaction around this region. Schlieren visualization is used to complement the experimental heat transfer data. To understand the effect of Mach number on pulsation and oscillation heat transfer measurements are carried out at Mach number of 8.2 in a shock tunnel. The suppression of the pulsation and oscillations fluctuations are obtained by attachment of a disc halfway along the length of the spike mounted in front of the flat-faced cylinder which is observed through a heat transfer and schlieren visualisation.